1. Field of the Invention
This invention relates to orthopedic screw systems used to fix surgical implants.
2. Description of Related Art
Orthopedic surgical implants, such as rods and plates, are often temporarily or permanently implanted in the body to repair fractures, and to gain fixation into bones to allow for the correction of various pathologies including deformity, instability and neoplastic conditions. Different types of implants have been created for different parts of the skeletal system and for different conditions. Regardless of type, most orthopedic implants have a common characteristic: they are anchored into bone by appropriate screws or other fasteners.
While the treatment of orthopedic conditions by implantation of orthopedic hardware is well established in patients with generally healthy bones, not all patients have healthy bones. If a bone screw is implanted in weakened or otherwise unhealthy bone, there is a greater risk that the bone screw will pull out or otherwise fail, in which case the implant secured by the bone screw may loosen, the orthopedic treatment may fail, and the patient may need more surgery to correct the problem.
Osteoporosis, a bone disease resulting in reduced bone mineral density and changes to the microstructure of the bones, is one of the major causes of weakened bones, although it is by no means the exclusive cause. At the time of writing, 20 million Americans suffer from osteoporosis, and 60 million are expected to suffer from it by the year 2020.
Osteoporosis can cause a number of different problems, including fractures of the hip and wrist and compression fractures of the vertebral column—the column of bones that encircles and protects the spinal cord. In particular, there are approximately 700,000 vertebral compression fractures in the United States per year.
Many of the problems associated with osteoporosis are themselves treatable. For example, compression fractures of the vertebrae are often treated with vertebral augmentation procedures, which are used to stabilize the spine. In a kyphoplasty, one type of vertebral augmentation procedure, the vertebral body is accessed through the vertebral pedicle, and catheters with balloons at their distal ends are inserted into the vertebral body. Once within the vertebral body, the balloons are inflated to create a cavity within the vertebral body, withdrawn, and bone cement is deposited into the cavity to fill it and, thus, to help restore the structural integrity of the vertebra and help treat the patient's pain. In another type of vertebral augmentation procedure, called a vertebroplasty, cement is injected into the vertebral body without first creating a cavity.
However, problems arise when a patient with weakened bones, due to osteoporosis, multiple myeloma, or some other condition, also has a diagnosis, such as scoliosis, infection, tumor, or degenerative stenosis, which would typically necessitate the implantation of orthopedic hardware. In those cases, many surgeons are reticent to implant corrective orthopedic hardware because of the increased risk of bone screw pull-out and instrumentation failure.
Therefore, it would be advantageous to have bone screws and surgical techniques that lower the risk of pull-out in patients with weakened bones.